The present invention relates to the filtering of control information in a disk drive servo control system. In particular, the present invention relates to a providing an oversampled digital filter for resonance compensation in connection with disk drive servo control systems.
Computer disk drives store information on magnetic disks. Typically, the information is stored on each disk in concentric tracks that are divided into servo sectors and data sectors. Information is written to or read from a disk by a transducer head, mounted on an actuator arm. The actuator arm is capable of moving to position the transducer head radially over the disk. Accordingly, the movement of the actuator arm allows the transducer head to access different tracks. The disk is rotated by a spindle motor at a high speed, allowing the transducer head to access different sectors within each track on the disk.
The actuator arm is interconnected to an actuator, such as a voice coil motor (VCM) to move the actuator arm such that the transducer head can access different tracks. Operation of the actuator is controlled by a servo control system. The servo control system generally performs two distinct functions: seek control and track following. In general, the seek function is initiated when a host computer associated with the disk drive issues a command to read data from or write data to a target track on a disk. Once the transducer head has been moved sufficiently close to the target track by the seek function of the control system, the track following function of the control system is activated to center and maintain the transducer head on the target track until the desired data transfers are completed.
Typically, the transducer head will oscillate about the center line of the target track for a period of time following the transition of the servo control system from the seek mode to the track following mode. In addition, while in the track following mode, adjustments to the position of the transducer head with respect to the center line of the target track are often required. Such small adjustments are required to correct drift in the position of the transducer head relative to the target track. The precise control of the position of the transducer head relative to a target track has become increasingly important as data densities in disk drives have increased.
In a typical disk drive, a digital signal processor or microprocessor is used to implement the servo control system. The digital control output of the digital signal processor or microprocessor is converted to an analog control signal and applied to the actuator (e.g., the voice coil motor) to position the transducer head relative to the target track. The processor used to implement the servo control system is often used to also implement the read/write channel of the disk drive to reduce the cost of the disk drive. Therefore, as more processing resources are devoted to implementing the servo control system, fewer of those resources are available for read and write operations, reducing the performance of the disk drive.
The mechanical actuator assembly, including the actuator, the actuator arm, and the transducer head, has flexible resonance modes. The phase and amplitude of the flexible resonance modes are difficult to control in mechanical manufacturing processes. Therefore, notch filters have been applied around the resonant frequencies to attenuate excitation of the actuator assemblies at those frequencies. However, the resonant frequencies may be greater than the Nyquist frequency of the servo control system.
The Nyquist frequency is defined as one-half the sampling frequency of a digital system. Signals having frequencies above the Nyquist frequency result in aliasing. Aliasing causes higher frequency signals present in the system to be reflected back to a frequency below the Nyquist frequency. With respect to a servo control system, the number of servo sectors containing positioning information used to correct the position of the transducer head during track following operations, the rate of rotation of the magnetic disks, and the speed and computational power of the microprocessor or digital signal process used to implement the read/write channel and the servo control system of the disk drive, determine the sampling rate of the servo control system. In general, the number of servo sectors utilized in a disk drive, and therefore the sampling frequency of the servo control systems, is limited, because portions of the disk devoted to servo sectors reduces the area of the disk available for user data, while the computational power of the processor is limited due to cost considerations.
One approach that has been proposed to remove unwanted signals at frequencies greater than the Nyquist frequency of the system is to apply a notch filter about the frequency at which the higher frequency signal is mirrored due to aliasing. However, the placement of a notch filter in the vicinity of the cross-over frequency of the closed loop servo control system results in high phase losses. As a result, the control system can become unstable, resulting in poor tracking of the target track by the transducer head.
Another approach to filtering frequencies from the control signal that can cause undesirable resonance in the actuator assembly is to apply a symmetrical, double sampling rate notch filter to the system. According to this approach, for each instance of position information acquired from the servo sectors, the digital notch filter provides two position control outputs. The output from the digital notch filter is applied symmetrically in that the additional control signal is equally spaced between the preceding and succeeding first control signals generated in response to each set of position information. Although such systems are effective in filtering high frequencies, a relatively large proportion of digital signal processor or microprocessor resources are required. In particular, because of the symmetrical spacing of the oversampled control outputs, processing overhead is increased because two interrupts must be generated for each sampling period in order to produce the oversampled control output. Accordingly, the overall performance of the disk drive is degraded, as relatively fewer resources in the digital signal processor or microprocessor are available for other functions, such as read and write operations.
It would be advantageous to provide a disk drive that is capable of providing an oversampled disk drive servo control system filter that did not require a large amount of processor overhead to implement. Furthermore, it would be advantageous to provide a disk drive having a servo control system that was capable of providing an oversampled control output to an actuator that did not require the execution of more than one interrupt service routine during a sampling period. Furthermore, it would be advantageous to provide a disk drive servo control system that provided an oversampled control output, that had low processor overhead requirements, that was inexpensive to implement, and that was reliable in operation.
In accordance with the present invention, a method and an apparatus for providing a variable rate oversampling digital filter for resonance compensation in disk drive servo control systems are provided. The present invention generally allows frequencies corresponding to resonance modes in actuator arm assemblies of a disk drive to be filtered, even though those frequencies are in excess of the Nyquist frequency of the servo control system. In addition, such filtering is achieved with relatively little digital signal processor or microprocessor overhead.
In accordance with an embodiment of the present invention, position information received from a servo sector is applied to a proportional-integral-derivative (PID) controller and the output of the PID controller is then applied to an oversampling digital filter. The oversampling digital filter provides a plurality of filtered outputs that can be applied to control of the actuator. Furthermore, the period of time over which a first filtered control output based on a first set of position data is applied to the actuator is not equal to the period of time over which a second filtered control output based on the first set of position information is applied to the actuator. This asymmetrical application of the first and second control outputs allows generation of the control output signals to be calculated during the same interrupt service routine. Accordingly, two outputs may be provided for each set of position data, without requiring the digital signal processor to service two interrupts during a sampling period.
According to another embodiment of the present invention, a first filtered control output, is determined during a first interrupt service routine using position data received from a first servo sector. The first filtered control output is provided to an amplifier during the interrupt service routine. A second filtered control output is then initiated during the same interrupt service routine, the second filtered output having been determined by the same input signal as the first one received. The period over which the first and second filtered control outputs are provided to the actuator are unequal.
According to still another embodiment of the present invention, at least one filtered control output initiated during each sampling period of the servo control system is proportioned. The control output may be proportioned so that the asymmetric control outputs initiated during a single interrupt service routine according to the present invention provide an amount of control energy that is equal to the amount of control energy that would be provided to the actuator by a symmetric oversampling control system. According to such an embodiment of the present invention, the proportioning of at least one of the asymmetric control outputs allows the control system to provide desirable frequency and phase response characteristics, while reducing digital signal processor overhead by allowing all control outputs initiated during a single sampling period to be initiated using a single interrupt service routine.
According to an embodiment of the present invention, the servo control system of the present invention incorporates a proportional-integral-derivative (PID) controller. The output of the PID controller is provided to an oversampling, asymmetric filter. In accordance with an embodiment of the present invention, the output from the PID controller is provided to the filter at twice the rate at which position data is received from the servo sectors (i.e. at twice the sampling rate). According to another embodiment of the present invention, the controller provides position data to the filter at a rate equal to the sampling rate, and the filter calculates two outputs for each set of position information received from the controller. The servo control system may additionally include an amplifier. The output from the filter to the amplifier may be in the form of an analog signal, or in the form of a digital signal that the amplifier converts to an analog signal. An analog control signal is then provided to the actuator. In general, the servo control system may include a sample and hold circuit, to provide the commanded output level until a next filtered output is received. The filter may be in the form of digital impulse response filter, and may act as a low pass or a notch filter.
Additional advantages of the present invention will become readily apparent from the following discussion, particularly when taken together with the accompanying drawings.